1. Field of the Invention
The present invention relates to a synchronization system for use in a digital transmission system in which an analog signal such as voice signal is converted into a digital signal and then transmitted.
2. Description of the Prior Art
As the digital transmission system in which an analog signal such as voice signal is converted into a digital signal and then transmitted, hitherto there is generally used a system in which the analog signal is converted into a 64 K bit/sec of PCM signal and then transmitted.
There have also been developed various types of high efficiency encoding schemes which are contemplated to transmit the voice and the like at lower bit rate. According to the CCITT (International Telegraph and Telephone Consultative Committee), ADPCM (Adaptive Differential PCM) scheme, in which an analog signal having 3.4 kHz of bandwidth is compressed to 32 k bit/s, is standardized in Recommendation G. 721, and further SB-ADPCM (Sub-band ADPCM) scheme, in which an analog signal having 7 kHz of band is compressed to 64 k bit/s, is standardized in Recommendation G. 722.
Now, in a case where ADPCM signals are transmitted, including ADPCM signals according to both the Recommendations (under the circumstances, also including SB-ADPCM signals), it is difficult for a decoder to reliably decode, if the decoder cannot discriminate which 4 bits or 8 bits among continuous digital signals or a bit stream are of data constituting one sample. In other words, it is necessary to accurately synchronize the ADPCM signals with synchronous input signals.
Hitherto, as methods of synchronization, there have been employed the following two methods. Now it is noted that for the purpose of an exemplary explanation of the conventional methods of the synchronization, description will be made on the ADPCM signals according to CCITT Recommendation G. 721.
FIG. 3 is a diagram for an explanation of a first method of synchronization. According to the first method of synchronization, information for synchronization such as a frame pattern is inserted into the ADPCM signal which is delivered between an encoder and a decoder. The frame pattern comprises a plurality (m, a natural number) of bits. The encoder distributes the m bits of the frame pattern to the LSB (Least Significant Bit) position of the samples separated from each other with the pitch of n samples, which are in turn transmitted. The decoder determines whether a pattern, which is formed by m bits separated from each other with the pitch of (4.times.n) bits, coincides with a predetermined frame pattern. The synchronization is established so as to have the received ADPCM signal separated into four bits coinciding with the frame pattern.
FIG. 4 is a diagram for the explanation of a second method of synchronization. According to the second method of synchronization, as shown in part (A) of FIG. 4, the ADPCM signal is transmitted, without any conversion, as it is. In addition, synchronous input signals or frame synchronous signals shown in part (B) of FIG. 4, each being significant during a period of MSB (Most Significant Bit) of the associated sample of the ADPCM signal, are delivered between an encoder and a decoder so that the synchronization can be obtained.
The first method of synchronization mentioned above has such an advantage that there are no signals such as synchronous input signals to be transmitted other than the digital transmission signals or ADPCM signals to be delivered between the encoder and the decoder. It has however such a disadvantage that there occurs deterioration on a decoded signal owing to insertion of the frame pattern into part of the digital transmission signal.
The second method of synchronization mentioned above has such an advantage that the digital transmission signals or ADPCM signals can be reliably transmitted, it has such a disadvantage that the synchronous input signals or frame synchronous signals in addition to the digital transmission signals have to be transmitted.